A conventional gas turbine engine includes a turbine having a plurality of circumferentially spaced rotor blades with tips thereof spaced radially inwardly from a stationary annular shroud for defining a clearance therebetween. The blade tip clearance should be as small as possible for minimizing leakage of combustion gases around the blades for obtaining improved efficiency of the turbine. However, the operating blade tip clearance must be large enough to accommodate differential thermal expansion and contraction between the rotor blades and the shroud to prevent undesirable rubs therebetween.
The blade tip clearance conventionally has different values at the different steady state operating conditions of the engine, and also has varying values during the various transient operating conditions of the engine which occur as the engine output power levels are varied. Transient blade tip clearance control is a significant concern since the differential thermal movement between the blade tip and the shroud typically has a minimum value, also referred to as a pinch point value which should be suitably large for reducing the possibility of blade tip rubs. However, with a suitably large pinch point, the blade tip clearance occurring at other times in the transient response, as well as during the steady state operation, is necessarily larger than the pinch point and, therefore, allows increased leakage of the combustion gases over the blade tips which decreases turbine performance.
Furthermore, although a gas turbine engine is typically axisymmetric, the temperatures in the environment of the turbine shroud are not necessarily uniform circumferentially about the engine centerline axis. For example, in one exemplary gas turbine engine including a recuperator, compressor discharge air is heated by the recuperator and channeled to the combustor through two circumferentially spaced recuperator conduits disposed near the top and bottom of the engine casing adjacent to the shroud of the high pressure turbine (HPT). Accordingly, the HPT shroud is positioned in an environment wherein the temperature varies substantially circumferentially, with relatively high temperature near the recuperator conduits and relatively low temperature therebetween. The blade tip clearance of the HPT, therefore, might vary circumferentially about the engine centerline axis for conventionally cooled shroud supports providing circumferentially uniform cooling air to the shroud. Accordingly, one object of the present invention is to provide a shroud support having more uniform circumferential cooling thereof for reducing circumferential variations in blade tip clearance.